The best test is doing a back pressure test. This is where you take out the o2 sensor and attach a pressure gauge. you can also measure the temperature of the inlet and outlet of the cat after running it a while. If the front is 400* or more hotter than the back, chances are that it is clogged.

You can also take out the o2 sensor upstream of the cat and see if you gain any power. It will be loud but it will tell the story!

Does the power issue continue throughout driving or just when you start out from a stop?

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Yes when the cruise is set on the highway it always feels like it is going headfirst into the wind, i still get 17 mpg on the highway even with the bigger tires on, but it just feels like it is always struggling. But I do know that if I put this up against my suburban and raced them and did a passing test my suburban would win hands down even with power and weight disadvantage. The mule also has a new fuel pump, filter, pressure reugaltor and O2 sensors